A polycarbonate is excellent in heat resistance, impact resistance and transparency, so that, in recent years, it has been widely used in many fields.
In a preparation process of the polycarbonate, many investigations have heretofore been done. Among these, a polycarbonate derived from an aromatic dihydroxy compound, for example, 2,2-bis(4-hydroxyphenyl)propane (hereinbelow, also referred to as “bisphenol A” or “BPA”) has been industrially produced by any preparation processes of the interfacial polymerization method or the melt polymerization method.
According to the interfacial polymerization method, the polycarbonate is produced from bisphenol A and phosgene, but poisonous phosgene must be used. Also, there remain the problems that the apparatus is corroded by a chlorine-containing compound such as by-produced hydrogen chloride and sodium chloride, and methylene chloride used as the solvent with a large amount, etc., and that removal of the impurities such as sodium chloride, etc., and remaining methylene chloride which cause effects on the polymer physical property, is difficult.
On the other hand, as a method for preparing a polycarbonate from an aromatic dihydroxy compound and a diarylcarbonate, for example, it has been known a melt polymerization method from long ago in which bisphenol A and diphenylcarbonate are polymerized in a melt state by transesterification, while removing the by-produced aromatic monohydroxy compound. The melt polymerization method has merits that it does not use a solvent, etc., different from the interfacial polymerization method.
With regard to the melt polymerization method, various attempts have been investigated to solve the problems that the polymerization degree is difficultly raised, etc. (for example, see Patent Documents 1 to 13). However, according to these methods, it cannot be said that the tasks to accomplish higher polymerization while maintaining good qualities of the polycarbonate inherently possessed can be sufficiently solved.
The present inventors have previously found out a novel process, as a process for obtaining an aromatic polycarbonate which can accomplish a rapid polymerization rate and gives good quality, in which end-capped terminals of the aromatic polycarbonate is connected with an aliphatic diol compound to elongate the chain (for example, see Patent Document 14; WO 2011/062220A pamphlet). According to this process, an end-capped terminals of the aromatic polycarbonates are linked to the aliphatic diol compound to elongate the chain, whereby an aromatic polycarbonate resin with a high polymerization degree having an Mw of about 30,000 to 100,000 can be produced within a short period of time.
The present inventors have also previously proposed a preparation process of a highly polymerized polycarbonate resin comprising the steps of highly polymerizing an aromatic polycarbonate prepolymer by reacting with a linking agent which comprises an aliphatic diol compound having a specific structure in the presence of a transesterification catalyst, and also removing at least part of a cyclic carbonate by-produced in the highly polymerizing step out of the reaction system (for example, see Patent Document 15; WO 2012/157766 A pamphlet).
The method of highly polymerizing the polycarbonate by using the linking agent comprising an aliphatic diol compound has merits that a structural unit derived from the linking agent is scarcely remained in the main chain of the obtained highly polymerized polycarbonate resin, the obtained polycarbonate resin has the same skeletal structure as that of the conventional polycarbonate obtained by the interfacial method so that the polycarbonate resin having excellent physical properties as those of the conventional ones can be obtained.
In such a preparation process of the highly polymerized polycarbonate resin, it has been desired to develop an industrial preparation method which can use the starting materials with good efficiency by recycling these materials, but in the preparation process of the polycarbonate resin, some proposals concerning reuse of by-produced phenol, etc., have been merely done (for example, see Patent Documents 16 and 17).